Module substrates are mechanically sufficiently stable carrier systems that preferably have an integrated electrical wiring and are provided for receiving one or preferably a plurality of interconnected chip components. Furthermore, it is also possible to realize passive components and circuits produced therefrom in the multilayer construction and thus to integrate them into the module. For incorporation into a circuit environment, the module substrate has solderable contacts at an underside, by means of which it can be soldered on a printed circuit board (PCB).
For the solderable contacts of ceramic module substrates, standard silver-containing pastes are printed on and fired into the ceramic. In order to produce a solderable surface, these are usually provided with a metallic coating, a gold layer usually serving as the topmost solderable layer.
Since the solderable contacts constitute the only structures formed on the underside of the module substrate, the fired or sintered pastes are usually provided with the metallic coating directly after firing in electrolytic or electroless methods. In this case, the electrically conductive paste is completely coated by the metallic coating.
In mechanical loading tests it has now emerged that the transition from the metallic coating to the ceramic constitutes a location in the module substrate that is subjected to severe mechanical loading and is, therefore, severely at risk of fracture. This stems from the fact that thermal and mechanical stresses usually occur after the soldering of the module substrate, the stresses preferably acting on the material coating.